01 - Prepare and bake the vanilla cake mix according to package instructions in a 9x13-inch baking dish. Allow to cool for 15 minutes before proceeding.
02 - Using the handle end of a wooden spoon, poke holes evenly across the entire surface of the cake, spacing approximately 1 inch apart.
03 - Combine mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a gentle simmer over medium heat. Whisk cornstarch with 2 tablespoons cold water until smooth, then stir into the berry mixture. Cook for 2-3 minutes while stirring constantly until the sauce thickens noticeably.
04 - For a smoother texture, press the berry sauce through a fine mesh sieve to remove seeds. Pour the warm sauce evenly over the cake, ensuring it fills the poked holes completely.
05 - Let the cake cool to room temperature, then refrigerate for a minimum of 1 hour to allow the berry flavors to fully infuse the cake.
06 - Beat heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form and the cream holds its shape.
07 - Spread the whipped cream evenly over the chilled cake surface. Arrange fresh mixed berries attractively on top and garnish with mint leaves if desired. Serve immediately or keep refrigerated until ready to serve.